PROJECT SUMMARY/ABSTRACT Chlamydia trachomatis genital infection is the most commonly reported infectious disease in the United States with no vaccine available. The rational design of a chlamydia vaccine requires improved understanding of the protective immune mechanisms within the female reproductive tract (FRT) mucosa. Although it is broadly accepted that CD4 T cells play a predominant role in protective immunity against Chlamydia FRT infection, the mechanisms underlying CD4 T cell-mediated immune protection remain to be poorly understood. This application proposes to develop a thorough understanding of the differentiation, anatomical distribution and maintenance of protective memory CD4 T cells and how they are stimulated to confer protective immunity against Chlamydia. We recently generated several Chlamydia-specific MHC Class II tetramers, which allow for the first time direct visualization of endogenous, antigen-specific CD4 T cells following Chlamydia FRT infection. Using these unique tools, we specifically propose to (i) examine the protective antigen-specific memory CD4 T cell responses that lead to fast Chlamydia clearance from the FRT, and (ii) identify the major antigen-presenting cells that stimulate the rapid protective memory CD4 T cell response in the FRT. We anticipate that identifying the protective aspects of memory CD4 T cell responses to Chlamydia reinfection and elucidating how immunological memory is maintained and reactivated in the host will provide important insights into the future design of an urgently needed chlamydia vaccine.